Primary growth is all about extending the roots and the shoots, but secondary growth actually only occurs in places that no longer grow in length, and secondary growth is all about expanding the circumference of the plant. Usually, we only see this in woody plants; there are some other types of plants that experience secondary growth, but we're just going to focus on woody plants as our example. Now, the meristem responsible for secondary growth is called a lateral meristem, and we're actually going to look at 2 types of lateral meristems, both of which are derived from the procambium, which you might remember is one of those primary meristems that gives rise to the vascular tissue system. Now, vascular cambium is one of those lateral meristems, and its job is to produce xylem and phloem. You might recall the procambium also produces xylem and phloem. We call those xylem and phloem produced by the procambium, primary xylem, and primary phloem. What the vascular cambium produces is known as secondary xylem and phloem. Now, the growth pattern here is kind of interesting. You see, the vascular cambium produces xylem and phloem both to the outside and inside of where it's located. I'm highlighting the vascular cambium as this layer of cells here in blue, that's our vascular cambium. Cells that it produces towards the outside of the plant, here is the outside, here's the inside. Cells that are produced towards the outside are secondary phloem, whereas, the cells produced towards the inside are secondary xylem. Now, what this results in is an actually pretty interesting growth pattern. You've probably seen a tree cut in half, and it has those growth rings. Those rings that, those concentric circles that continue all the way from the outside of the tree to the interior. Well, what's actually happening is the vascular cambium stays on, basically like the outer layer of the tree. The vascular cambium is going to stay towards the outside. It's going to leave behind lots of older xylem on the inside. Right? So these are all from the previous year's growth, as you can see. Whereas, this layer right here is the current year's growth. And again, these are older years seen in here. Well, these xylem that move or rather, due to the fact that the cork cambium continually produces these cells, but keeps moving towards the outside, or rather keeps its position on the periphery of the plant, we get this build-up of old xylem on the inner portions of the rings. We'll get back to those in a second. First, I want to quickly mention the cork cambium, that other lateral meristem, and this one is actually located even more towards the outside than the vascular cambium. We highlight the cork cambium here in red, it's this lateral meristem there, that's our cork cambium, and it's going to produce cells towards the outside that are called cork cells. These are non-living cells that are highly impermeable to gas and water and actually will form the structure known as bark. Now, I want to get back to those interior xylem. See, those interior xylem have a name that's much more common that you've probably heard of. It's called wood. Right? Wood is that porous structural tissue that's derived from those secondary xylem. It should be noted that it's only the outer layers of xylem that actually transport water. The inner layers accumulate various gums and resins to resist decay. And we actually have two names for these different regions of xylem. We have the heartwood, which, I'm going to jump out of the image here, is this darker internal area. So this is our heartwood. Then we also have the sapwood, which is this lighter-colored external layer. That's the sapwood. So, the heartwood is the layer that's accumulated all these gums and resins to resist decay. And it's no longer actively transporting anything. You know, effectively it is just structural support at that point. The sapwood, these lighter outer regions, those will have actively transporting xylem in them. Now, we also said that bark is formed from those cork cells. It actually is also made in part from the secondary phloem. Remember, the vascular cambium produces xylem to the inside and phloem to the outside. Well, some of those phloem, the non-active ones, will become bark, in addition with cork cells. So this layer, this bark because it has these cork cells in it, it's super impermeable to water and gas. But, trees have evolved, or say, woody plants have evolved, a way of dealing with that impermeability. They have these things called lenticels in their bark. Basically, it's a porous tissue that creates little openings to allow for gas exchange. And in this birch bark that you see right here, we have these horizontal lines that are very characteristic of birch bark, if you've ever seen it in the wild, those are lenticels. So these horizontal lines are our lenticels. Alright. That's all I have for this video. I'll see you guys next time.
Table of contents
- 1. Introduction to Biology2h 40m
- 2. Chemistry3h 40m
- 3. Water1h 26m
- 4. Biomolecules2h 23m
- 5. Cell Components2h 26m
- 6. The Membrane2h 31m
- 7. Energy and Metabolism2h 0m
- 8. Respiration2h 40m
- 9. Photosynthesis2h 49m
- 10. Cell Signaling59m
- 11. Cell Division2h 47m
- 12. Meiosis2h 0m
- 13. Mendelian Genetics4h 41m
- Introduction to Mendel's Experiments7m
- Genotype vs. Phenotype17m
- Punnett Squares13m
- Mendel's Experiments26m
- Mendel's Laws18m
- Monohybrid Crosses16m
- Test Crosses14m
- Dihybrid Crosses20m
- Punnett Square Probability26m
- Incomplete Dominance vs. Codominance20m
- Epistasis7m
- Non-Mendelian Genetics12m
- Pedigrees6m
- Autosomal Inheritance21m
- Sex-Linked Inheritance43m
- X-Inactivation9m
- 14. DNA Synthesis2h 27m
- 15. Gene Expression3h 20m
- 16. Regulation of Expression3h 31m
- Introduction to Regulation of Gene Expression13m
- Prokaryotic Gene Regulation via Operons27m
- The Lac Operon21m
- Glucose's Impact on Lac Operon25m
- The Trp Operon20m
- Review of the Lac Operon & Trp Operon11m
- Introduction to Eukaryotic Gene Regulation9m
- Eukaryotic Chromatin Modifications16m
- Eukaryotic Transcriptional Control22m
- Eukaryotic Post-Transcriptional Regulation28m
- Eukaryotic Post-Translational Regulation13m
- 17. Viruses37m
- 18. Biotechnology2h 58m
- 19. Genomics17m
- 20. Development1h 5m
- 21. Evolution3h 1m
- 22. Evolution of Populations3h 52m
- 23. Speciation1h 37m
- 24. History of Life on Earth2h 6m
- 25. Phylogeny2h 31m
- 26. Prokaryotes4h 59m
- 27. Protists1h 12m
- 28. Plants1h 22m
- 29. Fungi36m
- 30. Overview of Animals34m
- 31. Invertebrates1h 2m
- 32. Vertebrates50m
- 33. Plant Anatomy1h 3m
- 34. Vascular Plant Transport2m
- 35. Soil37m
- 36. Plant Reproduction47m
- 37. Plant Sensation and Response1h 9m
- 38. Animal Form and Function1h 19m
- 39. Digestive System10m
- 40. Circulatory System1h 57m
- 41. Immune System1h 12m
- 42. Osmoregulation and Excretion50m
- 43. Endocrine System4m
- 44. Animal Reproduction2m
- 45. Nervous System55m
- 46. Sensory Systems46m
- 47. Muscle Systems23m
- 48. Ecology3h 11m
- Introduction to Ecology20m
- Biogeography14m
- Earth's Climate Patterns50m
- Introduction to Terrestrial Biomes10m
- Terrestrial Biomes: Near Equator13m
- Terrestrial Biomes: Temperate Regions10m
- Terrestrial Biomes: Northern Regions15m
- Introduction to Aquatic Biomes27m
- Freshwater Aquatic Biomes14m
- Marine Aquatic Biomes13m
- 49. Animal Behavior28m
- 50. Population Ecology3h 41m
- Introduction to Population Ecology28m
- Population Sampling Methods23m
- Life History12m
- Population Demography17m
- Factors Limiting Population Growth14m
- Introduction to Population Growth Models22m
- Linear Population Growth6m
- Exponential Population Growth29m
- Logistic Population Growth32m
- r/K Selection10m
- The Human Population22m
- 51. Community Ecology2h 46m
- Introduction to Community Ecology2m
- Introduction to Community Interactions9m
- Community Interactions: Competition (-/-)38m
- Community Interactions: Exploitation (+/-)23m
- Community Interactions: Mutualism (+/+) & Commensalism (+/0)9m
- Community Structure35m
- Community Dynamics26m
- Geographic Impact on Communities21m
- 52. Ecosystems2h 36m
- 53. Conservation Biology24m
33. Plant Anatomy
Growth
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